Up to this time, various approaches to obtain a newly different color of the emitted light have been made by forming a thin-film EL element in which two or more light emitting layers, each having a different color of emitted light, are laminated together to change the color of the emitted light by the laminated layers.
For example, "Ryozo FUKAO et. al.: Electronic Information Communication Society Technical Study Report, Vol. 86, No. 368, p. 5, 1987" describes such a thin-film EL element as a "two-terminals type tunable color EL", and as a laminate of a green color light emitting layer formed of ZnS:TbF3 and a red color light emitting layer formed of ZnS:SnF3. It is reported herein that, when applying a voltage to such a element, the color of emitted light is changed from red to yellow-green by an increase in the voltage, as shown in FIG. 11.
Also, "S. TANAKA et. al.: Digest 1988 SID Int. Symp., P. 293, 1988" describes another thin-film EL element in which a light emitting layer formed of SrS:Ce,K emitting light of blue-green color and a light emitting layer formed of SrS:Eu emitting light of red color are laminated together. It is also reported therein that a change in the voltage causes the color of the emitted light to change.
However, when making a panel for dot matrix display by using such laminated type of thin-film EL elements mentioned above, the effective voltage applied to the light emitting layer depends on the position in accordance with thickness distributions of the light emitting layer and the insulating layer, so that the color of the emitted light can vary with the location. Also, a voltage drop by line resistance of the electrode causes the color of emitted light to change between the bottom and the tip of the electrode. For these reasons, a problem called "nonuniformity of color" has arisen, so that making a useful panel could not be achieved.
It is considered that the above-mentioned problems are caused by the formation of a high resistant layer where crystallinity is low, also called a "dead layer", between the light emitting layer and the insulating layer with the thickness being from approximately 1000 to approximately 2000 .ANG.. The "dead layer" generally occurs in a light emitting layer formed by conventional light emitting layer forming technique, such as EB (Electron Beam) evaporation method or sputtering method (e.g., see "H. SASAKURA et. al.: J. Appl. Phys. 52 (11), 6901, 1981").
When applying a voltage to a thin-film EL element which includes the conventional laminated type of light emitting layers mentioned above, each respective layer functions as independent thin-film EL elements. Such independent EL elements have "luminance--voltage" characteristics which are different from each other, thus causing the color of the emitted light to change in accordance with a change in the voltage.
For example, when the laminated light emitting strata has two layers, as shown in FIG. 12, it has a structure equivalent to that of a double circuit which includes two pairs of Zener diodes a and b, each pair being connected opposite to each other in series; two capacitors c connected in series, each being connected in parallel with the serially connected Zener diodes; and a capacitor d connected to one end of the two capacitors c.
On the other hand, up to the present, there have been various methods for obtaining full color display with a thin-film EL element. Of these, there are two typical types; one type uses a planar pattern formed of three kinds of materials each of which emits light of a respective one of the three primary colors, red (R), green (G) and blue (B) as shown in FIG. 13; the other type laminates such luminescent materials and decomposes the resulting mixed color emitted light by passing it through filters as shown in FIG. 14.
In FIG. 13, there are provided a glass substrate e, transparent electrodes d patterned on the glass substrate e, first and second insulating layers f and g, a segmented light emitting layer h in which each segment emits light of a respective one of the three primary colors and which are patterned between the insulating layers f and g, and a back plate i.
In FIG. 14, the same references as those of FIG. 13 indicate similar elements except a color filter k, and the light emitting layer h in FIG. 14 is formed by laminating three light emitting layers, each emitting a respective one of the three primary colors R, G and B.
However, the former, which is a patterned light emitting layer type, capable of full color display with the conventional thin-film EL element, has had such problems as the forming process being complicated, the light emitting layer being damaged during patterning, and the like.
Although the forming process is simple for the latter, which is a laminated light emitting layer type, the respective materials have different L--V characteristics. Further, the intensity of the electric field effectively applied to the intermediate light emitting layer is lower than that of each adjacent light emitting layer, so that other problems have arisen such that it was difficult to separate beams of light from the respective layers under a well-balanced condition.
In another method which has also been considered, white light, having a wide spectrum obtained from a single light emitting layer, such as SrS:Ce,Eu or the like, is separated by a color filter. However, efficient brightness can not be obtained from the light emitting layer formed of SrS:Ce,Eu and chemical stability of the base material SrS is worse.